In a CDMA (Code Division Multiple Access) mobile communication system, scrambling codes are used in communication between a Node-B (base station) and UE (User Equipment).
The following explanation regards scrambling codes.
There are two types of scrambling codes: scrambling codes for UL (Uplink) and scrambling codes for DL (Downlink). UL scrambling codes are used for identifying UE, and DL scrambling codes are used for identifying cells. A cell is an area in which Node-B provides service.
There are 8192 DL scrambling codes, and the scrambling codes are divided into 512 groups of 16 codes apiece. In other words, there are 16 scrambling codes from 0, 1, . . . , to 15 in each group, the 0th scrambling code being called the primary scrambling code and the other codes being called secondary scrambling codes. In each cell, the primary scrambling code is used without fail, and the secondary scrambling codes may be used in addition.
A UL scrambling code is made up by 24 bits. In other words, there are 16777216 UL scrambling codes. The physical channels of physical layers in UL are broadly divided between two types: PRACH (Physical Random Access Channels) and UL DPCH (Dedicated Physical Channels). The 3GPP (3rd Generation Partnership Project) standards specify that the 8192 UL scrambling codes from the 0th to the 8191th codes are to be used for PRACH (Non-Patent Document 1). In other words, the 8192nd and following UL scrambling codes are used for DPCH.
There are two parts in PRACH: the Preamble Part and the Message Part. The 3GPP standards specify that either the primary scrambling code that is used in the DL of the same cell or the scrambling codes of the same group (0, 1, . . . , 511) are to be applied to the UL scrambling code used in the PRACH Preamble Part (Non-Patent Document 1). The 3GPP standards (Non-Patent Document 2) further specify that Node-B reports to its own cell the number (0 . . . 15) of the scrambling code in the same group as the DL scrambling code of its own cell that is to be used in PRACH in a message of SIBS (System Information Block type 5) or SIB6 (System Information Block type 6). If the same DL scrambling code group is being used in an adjacent cell, UE will encounter difficulty in identifying the cell and will become unable to communicate normally with Node-B. As a result, Node-B basically does not use a DL scrambling code group that is the same as that of a neighboring cell and the UL scrambling code of a PRACH Preamble part is not duplicated between adjacent cells. It is determined in the 3GPP standards that in the PRACH Message Part, values that are shifted by 4096 from the UL scrambling code of the PRACH Preamble Part are to be applied as input to a code generator, whereby different codes are used even when the code identification numbers are identical (Non-Patent Document 1).
In UL DPCH, the 8192nd and succeeding UL scrambling codes among the 16777216 UL scrambling codes can be used as described hereinabove, and the UL scrambling code range that can be used is therefore broad. As a result, the potential for duplication of UL scrambling codes is low and assignment logic is therefore not specially determined in the 3GPP standards.